The present invention relates to compression and decompression of signals in a transceiver system of a wireless communication network and, more particularly, to compressing signal samples prior to transfer over a serial data link between a base station processor and one or more radio frequency (RF) units of the transceiver system.
Transceiver systems in wireless communication networks perform the control functions for directing signals among communicating subscribers, or terminals, as well as communication with external networks. The general operations include receiving RF signals, converting them to signal data, performing various control and signal processing operations on the signal data, converting the signal data to an RF signal and transmitting the RF signal to the wireless subscriber. Transceiver systems in wireless communications networks include base stations and distributed antenna systems (DAS). For the reverse link, or uplink, a terminal transmits the RF signal received by the transceiver system. For the forward link, or downlink, the transceiver system transmits the RF signal to a subscriber, or terminal, in the wireless network. A base station may also be called a base transceiver system (BTS), cell site, access point, Node B, or other terminology. A terminal may be fixed or mobile and may be a wireless device, cellular phone, personal digital assistant (PDA), personal computer or any device equipped with a wireless modem.
For this description, the term base transceiver system (BTS) will refer to the base station processor(s) and the RF unit(s) in communication with and under the control of the base station processor, including any type or length of data transfer link. This includes the traditional base station having the RF units collocated with the base station processor or on the antenna tower near the antenna. A DAS is another example of a BTS, although the RF units are remote from the base station processor.
The base transceiver systems of wireless communication networks must manage the increasing amounts of data required for offering new services to an expanding subscriber base. System design challenges include ensuring flexibility for evolving standards, supporting growing data processing requirements and reducing overall cost. The modular design approach for base stations provides the flexibility to meet these challenges. The components of modular base station designs include base station processors and RF units coupled by serial data links, comprised of copper wire or fiber optic cabling. The RF units include transmitters, receivers, analog to digital converters (ADCs) and digital to analog converter (DACs). Wire or fiber optic serial data links transfer the sampled signals between the RF units and the base station processor. The sampled signals may be centered at the RF or converted to an intermediate frequency (IF) or baseband prior to transfer over the data link. The base station processor includes functions for signal processing, control and communication with external networks.
Two industry standards for modular designs, the Open Base Station Architecture Initiative (OBSAI) and Common Public Radio Interface (CPRI), specify architectures for interconnection of RF modules and base station processors as well as data transfer protocols for the serial data links. The OBSAI standard is described in the documents, “OBSAI Open Base Station Architecture Initiative BTS System Reference Document”, Version 2.0, 2006, and “OBSAI Open Base Station Architecture Initiative Reference Point 3 Specification”, Version 4.0, 2007. The CPRI standard is described in the document, “CPRI Specification V3.0 Interface Specification”, 2006. Both architectures transmit/receive multichannel signal data and transfer multiplexed baseband signal data over the serial data link. Modular designs may not necessarily comply with CPRI or OBSAI.
The OBSAI standard describes architectures and protocols for communication between base station processors, referred to as baseband modules, and RF modules. Connection topologies for one or more baseband modules and one or more RF modules include mesh, centralized combiner/distributor and bridge modules. The OBSAI compliant serial data link connecting the baseband module and the RF module is referred to as the reference point 3 (RP3) interface. In systems where remote RF units (RRUs) are connected to a baseband module, the serial data link is referred to as the RP3-01 interface. Connection topologies for the baseband module and RRUs include point-to-point, chain, ring and tree-and-branch. The baseband module/RRUs configurations support distributed antenna systems.
The CPRI standard refers to radio equipment control (REC) for processing baseband signal data and the radio equipment (RE) that performs the RF processing for transmission of signals over the antenna. The REC and RE correspond to the base station processor and the RF unit, respectively. The CPRI standard specifies the serial interface and operations at the physical and data link layers. The serial data link between REC and RE, or between two REs, is a bidirectional interface with one transmission line per direction. Connection topologies between the REC and one or more REs include point-to-point, multiple point-to-point, chain, star, tree, ring and combinations thereof.
Distributed antenna systems (DAS) distribute signal data from a main antenna/RF resource to multiple remote antennas connected via Cat5 cable, coaxial cable or fiber optic links. In essence, a DAS can connect to a variety of wireless services and then rebroadcast those signals throughout the areas in which the DAS is installed. For example, a DAS can improve cell phone coverage within a building. A main transceiver and antenna on the roof of the building is connected by cable or fiber to multiple distributed antennas within the building. Every DAS has a “head end” into which source signals are combined for distribution to remote radio units. The DAS systems provide coverage in confined spaces such as high rise buildings, tunnels, railways, and airports. As defined by the DAS Forum of the Personal Communications Industry Association (PCIA), a DAS is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. The DAS antenna elevations are generally at or below the clutter level and node installations are compact. A digital serial data link connects the head end (base station) to remote radio units, or heads.
Base transceiver systems for wireless communication networks transfer large amounts of sampled signal data over the serial data links between the base station processor and the RF modules. The need to comply with evolving wireless communication standards, increase data volume and serve more subscribers, may require expensive hardware upgrades to transceiver systems, including increasing the number or capacity of serial data links and increasing the data processing capability of supporting subsystems. These requirements can conflict with constraints on transceiver systems, including physical size limitations, power consumption limitations and geographic restrictions.
Therefore, there is a need for increasing the capacity of serial data links and conserving the resources of base transceiver systems for base stations and distributed antenna systems. Compression of data prior to transfer over the serial data links enables the provider to meet these needs by increasing the capacity of existing data links, possibly eliminating or at least postponing, the need to upgrade the existing data links. Computationally efficient compression and decompression conserves computing resources. The OBSAI and CPRI standards do not disclose compressing signal samples prior to transfer over the serial data links. Therefore, there is also a need for providing compressing signal samples and formatting the compressed samples for compatibility with the data transfer protocols of the BTS.